(a) Technical Field
The present disclosure relates to a braking control method and system for an eco-friendly vehicle, and more particularly, to a braking control method and system for an eco-friendly vehicle which improves braking stability and regenerative braking efficiency through optimal distribution of front/rear wheel power of an all-wheel-drive (AWD) driving system while braking.
(b) Background Art
As is well known, when a vehicle which is driven using an electric motor, that is, an eco-friendly vehicle such as a pure electric vehicle (EV), a hybrid electric vehicle (HEV), or a fuel cell vehicle (FCV) applies a brake (e.g., when a brake is engaged), regenerative braking is performed. A regenerative braking system of the eco-friendly vehicle converts kinetic energy of the vehicle into electric energy and stores the energy in a battery while braking the vehicle and then reuses the energy to drive an electric motor when the vehicle is driven, to thus improve fuel efficiency of the vehicle.
In the vehicle in which the regenerative braking is performed, a regenerative braking co-operative control technique which adjusts a sum of a regenerative braking torque generated in the electric motor (a driving motor) and a frictional braking torque generated in a brake device to be equal to a driver demand braking torque during the regenerative braking is required. In particular, it is necessary to appropriately distribute an electric braking force caused by an electricity generating operation and a rotational resistance of the electric motor, that is, a regenerative braking force and a frictional braking force by a frictional braking device and thus, co-operative control between controllers needs to be appropriately performed.
In a general eco-friendly vehicle according to the related art, a hydraulic braking device is used as a frictional braking device and a driver demand braking force which is a target braking force to decelerate the vehicle is calculated based on a brake signal that corresponds to manipulation of a brake pedal, that is, a signal of a brake pedal sensor (BPS) based on manipulation of a brake pedal, and a regenerative braking force and a hydraulic braking force are distributed to satisfy a total braking force, using the driver demand braking force as the total braking force which needs to be generated in the vehicle.
When a regenerative braking torque and a hydraulic braking torque are determined by distributing the regenerative braking force by an electric motor and a hydraulic braking force by a hydraulic braking device, regenerative braking and hydraulic braking are adjusted to obtain distributed torque values. In addition, an electronic braking force distribution system (EBD) in the vehicle distributes braking force to front wheels and real wheels to maximize a braking force and an electric and hydraulic braking system (EHB) in an eco-friendly vehicle, such as an electric vehicle or a hybrid electric vehicle, distributes the driver demand braking force into the regenerative braking force by the electric motor and the hydraulic braking force by the hydraulic braking device.
In a general eco-friendly vehicle, when the electric motor which is a driving source is disposed in the front wheels or the rear wheels to use the front wheels or the rear wheels as driving wheels, the regenerative braking is performed only on the driving wheels. For example, when the front wheels are the driving wheels, after distributing the braking force into the front wheels and the rear wheels, the regenerative braking force and the hydraulic braking force are distributed into the front wheels on which the electric motor is disposed and the braking is performed on the rear wheels only by the hydraulic braking force device.
In the eco-friendly vehicle which performs only the front wheel regenerative braking, the electric motor configured to drive the vehicle is disposed on the front wheels. When the battery is charged by the electric motor to recover the energy, the regenerative braking force by the electric motor is applied only to the front wheels which are the driving wheels. However, in an all-wheel drive (AWD) vehicle in which the driving force from the driving source is distributed into the front wheels and the rear wheels, when the vehicle is driven, the driving force is distributed to the front wheels and the rear wheels. However, there is no strategy for distributing the regenerative braking force to the front wheels and the rear wheels when the AWD vehicle applies a brake.
In particular, with reference to the drawings, FIG. 1 is a view illustrating a configuration of a power train of a two wheel drive (2WD) transmission mounted electric device (TMED) hybrid electric vehicle in which an electric motor, that is, a driving motor is disposed in a transmission according to the related art. In a general TMED system, an engine clutch 12 is disposed between two driving sources which drive a vehicle, that is, an engine 11 and a driving motor 13 and the transmission 14 is disposed at an output side of the driving motor 13. The engine clutch 12 is engaged to or disengaged from the engine 11 and the driving motor 13 to connect or disconnect the power between the engine 11 and the driving motor 13 and the transmission 14 is configured to change the power of the engine 11 and the driving motor 13 to transmit the changed power to a driving shaft of the driving wheels 16.
In this configuration, when the engine clutch 12 is engaged, parallel mode driving in which the power of the engine 11 and the driving motor 13 is transmitted to the driving wheels 16 through the transmission 14 is performed. A starter-generator 17 which is connected to the engine to transmit the power is provided and the starter-generator 17 is configured to transmit the power while driving to the engine 11 to start the engine or generate electricity by a rotational force transmitted from the engine.
The battery 19 is charged by electric energy which is generated when the starter-generator 17 generates electricity. The battery 19 which is a power source (e.g., an electric power source) of the vehicle is connected to the driving motor 13 and the starter-generator 17 via an inverter 18 to be charged and discharged and the inverter 18 is configured to convert a direct current (DC) of the battery 19 into a three phase alternative current (AC) to apply the current, thereby driving the driving motor 13 and the starter-generator 17. Differently from the 2WD vehicle, a transfer case is provided in an AWD vehicle and the driving force transmitted from the driving source is distributed into the front wheels and the rear wheels through the transfer case.
In other words, for the transfer case, an electronic transfer is used which is provided at the output side of the transmission to distribute the driving force transmitted through the transmission into the front wheels and the rear wheels based on a power distribution rate determined in accordance with a driving condition of the vehicle and to vary the driving force distribution to the front wheels and the rear wheels to be 0 to 100%.
A regenerative braking distribution control technique of the related art is developed in consideration of the 2WD vehicle as illustrated in FIG. 1 and the driving motor 13 is connected only to the front wheels or the rear wheels which are the driving wheels 16, and thus, braking force distribution to the front wheels and the rear wheels and the regenerative braking force and hydraulic braking force distribution to the driving wheels are sequentially and independently performed. For example, regenerative braking distribution of a 2WD hybrid electric vehicle in which the front wheels are driving wheels will be described herein below.
First, when a brake signal based on manipulation of a brake pedal is generated and the driver demand braking force, that is, a total braking force required in the vehicle is determined based on the brake signal, an electronic control unit (ECU: which may be the ABS and the ECU)) of the EBD is configured to distribute the braking force into the front wheels 16 and the rear wheels as described below, using driving state information of the vehicle and vehicle weight distribution information due to the number of passengers and a loading weight.Total braking force=Front wheel braking force+Rear wheel braking force
Further, when the braking force is distributed into the front wheels and the rear wheels, the electronic control unit of the EHB is configured to distribute the braking force of the front wheels 16 which are driving wheels into the regenerative braking force and the frictional braking force (hydraulic braking force) as described below, based on information regarding the motor state and the battery state.Front wheel braking force=Regenerative braking force+Frictional braking force
As a result, with respect to the rear wheels (not illustrated in FIG. 1) to which a driving system of the vehicle is not connected, control of the frictional braking device (e.g., hydraulic braking device) is performed to generate a rear wheel braking force only using frictional braking and with respect to the front wheels 16, the regenerative braking control of the motor and control of the frictional braking device are performed under co-operative control of a controller within the vehicle which is known to generate the regenerative braking force and the frictional braking force which satisfy the distributed front wheel braking force.
However, in the AWD hybrid electric vehicle, the power distribution rate is determined based on information collected from the vehicle while the vehicle is driven and the transfer case is operated based on the determined power distribution rate only to adjust the distribution of the driving force (e.g., power distribution rate). However, there is no front/rear wheel distributing technique of a substantially large braking force, such as regenerative braking, and specifically, no AWD power distributing technique which satisfies both the front/rear wheel braking distribution and regenerative braking/frictional braking distribution.
When the existing driving power-based AWD vehicle power distributing strategy in regenerative braking of the AWD vehicle through front wheels and rear wheels is applied to the regenerative braking, an area where the regenerative braking recovery efficiency is decreased is generated, which will be described with the following example. An example in which the braking is performed in a state where the AWD power distribution rate of the vehicle is determined to be 100% in the front wheels will be described.
First, when the vehicle brake is engaged, a total braking force which is demanded by a driver is distributed to target braking forces for the front wheels and the rear wheels, that is, a front wheel braking force 21 (Front) and a rear wheel braking force 22 (Rear) using information such as a vehicle driving situation and vehicle weight distribution, as illustrated in FIG. 2 (step 1).
Further, the EHB is configured to distribute the total braking force into the regenerative braking force 23 (Regenerative) and a frictional braking force 24 (Hydraulic) based on information regarding a motor state and a battery state (see step 2 of FIG. 2 of the related art). In particular, when the AWD power distributing method which does not consider the regenerative braking recovery efficiency is applied, in a state where the current power distribution rate of the transfer case is 100% in the front wheels, the regenerative braking force is also applied only to the front wheels. Therefore, the regenerative braking force distributed in step 2 may exceed the front wheel braking force distributed in step 1.
However, the regenerative braking force should not exceed the distributed front wheel braking force. Therefore, when the regenerative braking force determined in step 2 exceeds the front wheel braking force, the regenerative braking amount requires limitation when controlling the braking of the vehicle (step 3). As a result, a regenerative braking loss that corresponds to an amount which exceeds the front wheel braking force excluding effective regenerative braking corresponding to the front wheel braking force should be caused.
When 100% of the regenerative braking force which is limited in a state where the power distributing rate of the transfer case is 100% in the front wheels is applied to the front wheels, only the frictional braking force (hydraulic braking force) is applied to the rear wheels. In particular, even though the regenerative braking may be additionally used, the regenerative braking amount to the front wheels is limited and when the braking is controlled, the frictional braking amount (hydraulic braking amount) for the rear wheels needs to be increased compared to the amount determined in step 2 to satisfy the total braking force and the rear wheel braking force in step 1.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.